Method of crucible-free zone melting crystalline rods,especially of semiconductive material



N 1, 1969 w. KELLER 3,477,811

METHOD OF CRUCIBLE-FREE ZONE MELTING CRYSTALrLINE RODS, ESPECIALLY 0F SEMICONDUCTIVE MATERIAL Filed July 11, 1966 2 Sheets-Sheet 1 Fig.7 Fig.8

NOV. 11, 1969 w, KEL

METHOD OF CRUGIBLE-FREE ZONE MELTING CRYSTAL-LINE. RODS, ESPECIALLY OF SEMICONDUGTIVE MATERIAL Filed July 11, 1966 2 Sheets-Sheet Fig. 9

United States Patent 8 Int. (11.11011 i7/10, 17/20 US. Cl. 23-301 Claims ABSTRACT OF THE DISCLOSURE A method of zone melting a semiconductor rod wherein the rod is supported vertically by end holders located in the vertical axis of the rod, and a molten zone is formed in the rod by an annular heating device surrounding and spaced from the rod and being relatively displaced along the rod axis, whereby the molten zone is passed along the rod in the direction away from one of the end holders which one is meanwhile rotated, the end holders being relatively moved in an axial direction with respect to one another, includes reciprocating the one end holder transversely to the vertical axis of the rod and the annular heating device so as to alternately draw the molten zone toward the periphery of the rod portion located between the one end holder and the molten zone at most over a distance limited by the propensity of the molten material to drip from the molten zone, and withdraw the molten zone from the periphery of the rod portion.

My invention relates to method of crucible-free zone melting crystalline rods, especially of semiconductive ma terial.

In my Patent No. 3,414,388, issued Dec. 3, 1968, I have disclosed and claimed a method of crucible-free zone melting a crystalline rod, especially of semiconductive material, which is vertically clamped at its ends, by end holders, of which one at least is rotated about its vertical axis. The end holders are moved in the direction of the rod axis relatively to each other and to an annular heating device surrounding the rod and forming a melting zone therein, the relative speeds being adjusted so that the rod portion resolidifying from the melt is formed to a specific diameter. The resolidifying rod portion is laterally displaced relatively to the heating device, i.e. transversely to the vertical rod axis, whereby the melting zone is drawn toward the rim of the freezing rod portion, but only so far at most as to avoid dripping of the molten material therefrom.

It is an object of the invention in the instant application to provide an improvement over the method described in my aforementioned copending application and, more particularly, to impart to the resolidified rod an improved uniform resistivity over the cross section thereof.

With the foregoing and other objects in view, I provide a method according to this invention, wherein subsequent to the steps of the method disclosed in my aforementioned issued patent, the holder of the resolidifying rod portion is laterally displaced in the opposite transverse direction, whereby the melting zone is drawn away from the rim of the resolidified rod portion. In accordance with a further feature of my invention, the holder of the resolidifying rod portion is repeatedly shifted or reciprocated laterally or transversely to the vertical axis of the rod. It has been found particularly useful to repeat the aforementioned lateral or transverse to-and-fro movement several times during one pass of the melting zone.

3,477,811 Patented Nov. 11, 1969 The repetition can be carried out periodically. This method and its improvements are suitable especially for the zone melting of silicon rods.

Other features which are considered as characteristic for the invention are set forth in the appended claims.

Although the invention is illustrated and described herein as method of crucible-free zone melting crystalline rods, especially of semiconductive material, it is nevertheless not intended to be limited to the details shown, since various modifications and changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalence of the claims.

The method of this invention, however, together with additional objects and advantages thereof, will be best understood from the following description when read in connection with the accompanying drawings, in which:

FIGS. 1 to 5 are elevational views, partly broken away and partly in section, showing various phases during the carrying out of the method of the invention;

FIG. 6 is an elevational view, partly broken away and partly in section, showing a phase comparable to that of FIG. 4 during the carrying out of a variation of the method of this invention.

FIGS. 7 and 8 are plots of the local resistivity distribution over the cross section or diameter of the crystalline rod produced by conventional zone melting methods; and

FIG. 9 is a schematic elevational view of apparatus for carrying out the method in accordance with this invention.

Referring now to the drawings and first to FIG. 1, there is shown a semiconductor rod 2, to the lower end of which a seed crystal 5 is fused with the aid of an induction heating coil 3 energized by a suitable high frequency current. A melting zone 4 is thus produced in the rod 2 and can be passed in the longitudinal or vertical direction of the rod, as shown in FIG. 1, by moving the heating coil 3 upwardly or, when the heating coil is stationary, by moving the holders of the crystalline rod 2 downwardly. The seed crystal 5 may be a monocrystal so as to effect the growth of a monocrystalline rod 2. This monocrystalline seed 5, as well as the recrystallized rod portion are rotated about their vertical axis, as shown in FIG. 1, the lower holder for instance being driven by means of an electric motor. At the instant shown in FIG. 1, the melting zone has reached the point of development in which the transition from the thin seed crystal to a larger diameter of the lower rod portion has been achieved, it being noted however that this larger diameter of the lower rod portion does not exceed the inner diameter of the heating coil 3.

In FIG. 2, the further steps of my method are indicated by the arrows. The seed crystal 5 is not only moved downwardly with respect to the heating coil 3 which is assumed to be stationary, but is also simultaneously moved laterally or in a direction transverse to the vertical or longitudinal axis of the rod 2, for instance, to the right-hand side of FIG. 2, so that the lower part of the melting zone 4 is also stretched or drawn toward the right. As the seed crystal 5 rotates about its axis, the material crystallizing from the melt grows or solidifies substantially symmetrically to the rotary axis of the lower rod holder (not shown in FIG. 2). Thereby the diameter of the resolidifying lower rod portion exceeds the inner diameter of the heating coil 3. The upper rod portion 2 is moved downwardly from above at a suitable rate. It can also be rotated about its longitudinal axis.

In FIG. 3, a later phase of the method is shown in which, by further displacing the seed crystal 5 toward the right-hand side a further enlargement of the diameter of the lower rod portion 2a up to a desired value is achieved. At the instant shown in FIG. 3, the direction of the lateral displacement of the lower holder is being reversed, and this holder is now beginning to be moved from the right toward the left-hand side of the figure. Since the lower holder is steadily rotating about its axis at the same time, it would be sufiicient to move it back laterally only to a middle position in which both rod portions are coaxially aligned. The lateral displacement can be continued beyond the middle position, however, until the melting zone extends to the opposite rim of the lower rod portion 2a, as shown in FIG. 4, wherein the lower rod holder which holds the seed crystal 5 has reached the left-hand limit of its displacement. As the horizontal arrow indicates in FIG. 4, during the continued movement of both rod portions 2, 2a in downward direction of the rod axis, the direction of lateral displacement is again reversed.

In FIG. 5, a further phase is reached, in which the direction of the lateral movement is reversed for a third time.

Following is an example of the method of my invention in which it is assumed that the diameter of the rod is enlarged to twice its original value so that the rod cross section becomes four times as large as initially. Consequently, the upper rod portion 2 must be moved downwardly with respect to the heating coil 3 with a speed four times as great as the speed with which the lower rod por tion 2a is being drawn away downwardly from the heating coil. In this example, the lower rod portion 2a was moved downwardly with a speed of 2 mm./min., while the upper rod portion 2 was moved downwardly with a speed of 8 mm./min. The speed of rotation of the lower rod portion was 20 r.p.m. but may be within the range of about 5 to 100 rpm. The lateral reciprocatory movement of the lower holder was carried out periodically 5 to 20 times per minute.

In FIG. 6, a variation of the method of my invention is shown, in which the recrystallizing rod portion 12a above the heating coil 13 is drawn away in an upward direction, and also the rod portion 12 is moved upwardly relatively to the heating coil 13. The melting zone 14 can be maintained at the upper rod portion 12a due to surface tension and because of the large area of cohesion if it is particularly short in length, i.e. in the direction of the longitudinal axis of the rod. For this purpose, heating by a flat induction coil with helical windings is particularly advantageous.

In FIG. 7, there is shown a plot of the resistivity of the recrystallized rod across the cross section thereof as achieved after several repeated passes of the melting zone through the rod in accordance with prior art methods of crucible-free zone melting. It is desirable, of course, that the most uniform distribution of resistivity be obtained. When no special measures are taken, the curve shown in FIG. 7 is produced, wherein the resistivity p, beginning at one peripheral location of the rod denoted by O in FIG. 7 remains uniform at first along the diameter of the rod, until about the center of the rod denoted by r,

i.e. one radial distance of the rod, when it then clearly decreases by several percent. Thereafter, the resistivity increases symmetrically with respect to the center of the rod up to its original value, which then remains constant up to a diametrically opposite peripheral location D of the rod.

FIG. 8 is presented for the purpose of comparison with FIG. 7, and shows the local resistivity across the cross section of a rod produced by the method of my aforementioned copending application. Since the melting zone in the described embodiments is formed in a semiconductive rod about half the thickness of the semiconductive rod which is to be grown, there is produced in the grown rod 2a a local resistivity distribution which corresponds to a doubling of the curve in FIG. 7. In the case shown in FIG. 8, the decreases in the resistivity are not located in the center r of the rod cross section but at a location which is a distance /zr from the initial peripheral location 0 or from the center r of the rod. This is the annular zone of the solidifying rod portion 2a which, due to the rotation of rod portion 2a, passes through the extension of the axis of the annular heating device. The diagrams provide only a qualitative, not a quantitative, illustration of the observed differences in the local resistivities.

In contrast to the foregoing, no such differences in resistivity are produced if the lateral or transverse displacement of the holder of rod portion 2a does not cease after reaching the final posiiton D, but is reversed toward the starting position 0. It can be readily seen that repeated transverse reciprocation causes the particles of the material of decreased local resistivity crystallizing from the melt to be distributed almost over the entire cross section of the resolidifying rod portion, so that virtually no local differences in resistivity can be detected. Particularly by shifting the rod holder transversely back and forth so that the center of the rod portion 2 reaches the peripheral limits of the rod portion 2a the specific resistivity can be made substantially uniform over the entire rod cross section, without exhibiting any noticeable deviation.

FIG. 9 is a diagrammatic view of apparatus for carrying out the method of the invention in this application, and corresponds substantially to the apparatus described in my aforementioned copending application.

A rod comprising an upper rod portion 2 and a lower seed crystal portion 5 is vertically supported by the end holders 101, 102. A slider 103, displaceable on a rotary spindle 104, has an extension abutting the holder 101 so as to be able to displace the holder 101 and the rod portion 2 in either vertical direction depending on the direction of rotation of a reversible motor M A reversible motor M is supported on the extension 105 for rotating the holder 101 and the rod portion 2. The motor M is supported on a base 106 of the apparatus. The holder 102 of the seed crystal is rotatable by a motor M which has a displaceable shaft at one end of which the holder 102 is secured. The motor M is fastened to a slide 107 which is horizontally displaceable by a rack and pinion mechanism driven by a reversible motor M mounted on the base 106 whereby the holder 102 and the seed crystal 5 are displaceable in reciprocatingly opposite directions transverse to the vertical axis of the rod. A motor M also mounted on the base 106-, drives a rotary spindle 108 provided with a spindle head that is in engagement with the displaceable shaft of the motor M for vertically displacing the holder 102 and the seed crystal 5 being formed into a lower rod portion. The horizontal displacement distances of the seed crystal 5 and the holder 102 are relatively small so that the relatively wide abutting surface of the head on the spindle 108 engages the displaceable shaft of motor M in all of the possible horizontally displaced positions of the slide 107 and the motor M secured thereto. An induction heating coil 3, preferably with a fiat winding surrounds and is spaced from the molten zone 4 of the rod portion 2 and is vertically displaceable by a slider 109 and a rotary spindle 110 driven by a motor M which is mounted on the base 106.

Of course, as in accordance with the apparatus described in Patents Nos. 2,972,525; 2,992,311 and 3,030,194, the apparatus of my invention is located in a vacuum or protective gas atmosphere.

I claim:

1. A method of zone melting a semiconductor rod wherein the rod is supported vertically by end holders located in the vertical axis of the rod, and a molten zone is formed in the rod by an annular heating device surrounding and spaced from the rod and being relatively displaced along the rod axis, whereby the molten zone is passed along the rod in the direction away from one of the end holders which one is meanwhile rotated, the end holders being relatively moved in an axial direction with respect to one another, which comprises reciprocating said one end holder transversely to the vertical axis of the rod and the annular heating device so as to alternately draw the molten zone toward the periphery of the rod portion located between the one end holder and the molten zone at most over a distance limited by the propensity of the molten material to drip from the molten zone, and withdraw the molten zone from the periphery of the rod portion.

2. A method according to claim 1, wherein the one end holder is shifted in opposite directions transversely to the vertical axis of the rod.

3. A method according to claim 2, wherein the one end holder is periodcially displaced in opposite directions transversely to the vertical axis of the rod during one pass of the melting zone through the rod along the axis thereof.

4. A method according to claim 3, wherein the lateral reciprocatory movement of the one end holder is carried out periodically 5 to 20 times per minute.

5. A method according to claim 1, wherein a zone pass is initiated by moving the said end holders in the direction y eter larger than the inner diameter of the annular heating device, then, after forming the rod portion to the specific cross section, maintaining the said increased speed substantially constant.

References Cited UNITED STATES PATENTS 3,036,892 5/1962 Siebertz 23-273 3,159,459 12/1964 Keller 23-273 NORMAN YUDKOFF, Primary Examiner 

